Cone beam computed tomography (CBCT) scanners are well known and produce useful images of the interior structure of patients. They are invaluable as a diagnostic tool, and can also be used in conjunction with radiotherapeutic apparatus to produce realtime positional verification of organ location and even realtime guidance of the therapeutic radiation.
Such scanning does however meet with difficulties if the patient is not still. The three-dimensional tomograph is computed from a number of two-dimensional images, and the assumption must be made that the images are of an identical structure. If the patient (or parts of the patient) have moved between images then this results in degradation of the tomography and/or image artifacts. Such movement is of course inevitable, in the form of respiration and cardiac cycles.
Generally, improvements in the apparatus that allow a higher frame rate are regarded as desirable. These allow more images to be collected in a shorter time, resulting in an improved three dimensional tomography and/or reduced time demands on the patient.
To overcome the issue of respiration artifacts, we have proposed CBCT scanning that is correlated with the respiration cycle. This can be done either by detecting the respiration cycle and gating the scanner accordingly, or by scanning the patient and ascertaining the cyclical phase of a specific image from the image content. WO2004/06464 and WO2004/066211 describe such systems and a suitable algorithm for determining the phase of a specific image. This allows images of the “wrong” phase to be discarded prior to computation. Such respiration correlated CBCT (RCCBCT) allows good quality images of structures close to the lungs and/or diaphragm to be obtained.